Droplet-on-demand ink jet printing systems eject ink droplets from print head nozzles in response to pressure pulses generated within the print head by either piezoelectric devices or thermal transducers, such as resistors. The ejected ink droplets, commonly referred to as pixels, are propelled to specific locations on a recording medium where each ink droplet forms a spot on the recording medium. The print heads have droplet ejecting nozzles and a plurality of ink containing channels, usually one channel for each nozzle, which interconnect an ink reservoir in the print head with the nozzles.
In a typical piezoelectric ink jet printing system, the pressure pulses that eject liquid ink droplets are produced by applying an electric pulse to the piezoelectric devices, one of which is typically located within each one of the inkjet channels. Each piezoelectric device is individually addressable to enable a firing signal to be generated and delivered for each piezoelectric device. The firing signal causes the piezoelectric device receiving the signal to bend or deform and pressurize a volume of liquid ink adjacent the piezoelectric device. As a voltage pulse is applied to a selected piezoelectric device, a quantity of ink is displaced from the ink channel and a droplet of ink is mechanically ejected from the nozzle, commonly called an inkjet or jet, associated with each piezoelectric device. The ejected droplets are propelled to pixel targets on a recording medium to form an image on an image receiving member opposite the print head. The respective channels from which the ink droplets were ejected are refilled by capillary action from an ink supply.
In some phase change or solid ink printers, the image receiving member is a rotating drum or belt coated with a release agent and the ink medium is melted ink that is normally solid at room temperature. The print head ejects droplets of melted ink onto the rotating image receiving member to form an image, which is then transferred to a recording medium, such as paper. The transfer is generally conducted in a nip formed by the rotating image member and a rotating pressure roll, which is also called a transfix roll. The pressure roll may be heated or the recording medium may be pre-heated prior to entry in the transfixing nip. As a sheet of paper is transported through the nip, the fully formed image is transferred from the image receiving member to the sheet of paper and concurrently fixed thereon. This technique of using heat and pressure at a nip to transfer and fix an image to a recording medium passing through the nip is typically known as “transfixing,” a well known term in the art, particularly with solid ink technology.
Ink jet printers are capable of producing either simplex or duplex prints. Simplex printing refers to producing an image on only one side of a recording medium. Duplex printing produces an image on each side of a recording medium. In duplex printing, the recording medium passes through the nip for the transfer of a first image onto one side of the recording medium. The medium is then routed on a path that presents the other side of the recording medium to the nip. By passing through the nip again, an image is transferred to the other side of the medium. When the recording medium passes through the nip the second time, the side on which the first image was transferred is adjacent to the transfix roller. Release agent that was transferred from the image receiving member to the recording medium may now be transferred from the first side of the recording medium that received an image to the transfix roller. Thus, a duplex print transfers release agent to the transfix roller and multiple duplex prints may cause release agent to accumulate on the transfix roller.
Additional release agent may be applied to the transfix roller if the transfix roller comes into contact with the image receiving member during periods when there is no recording medium in the nip. The amount of release agent on the transfix roller may reach a level that enables release agent to be transferred from the transfix roller to the back side of a recording medium while an image is being transfixed to the front side of the recording medium. If a duplex print is being made, the back side of the recording medium, which receives the second image, now has release agent on it. The release agent transferred to the back side of the recording medium may interfere with the efficient transfer of ink from the image receiving member to the back side of the recording medium. Consequently, ink may remain on the image receiving member rather than being transferred to the recording medium. This inefficient transfer of ink may subsequently produce an image in which partial or missing pixels are noticeable. This phenomenon is known as image dropout. Additionally, ink remaining on the image receiving member may require the image receiving member to undergo a cleaning cycle.
To aid in the transfer of ink from the image receiving member to the back side of a recording medium, some printers perform the printing process using a printing process phasing or timing sequence that prevents the transfix roller from contacting the image receiving member. This printing process timing sequence minimizes the release agent on the transfix roller and thus minimizes the amount of release agent that may be transferred to the surface of the recording media. Use of a printing process timing sequence of this type, however, reduces printer throughput during duplex printing operations. Therefore, performing duplex printing in a manner that improves throughput without subjecting image quality to dropout and the like is useful.